1. Field of the Invention
This invention relates to global positioning systems and more particularly to techniques for improving the acquisition time of such systems.
2. Prior Art
A Satellite Positioning System (SATPS) is a system of satellite signal transmitters, with receivers located on the Earth's surface or adjacent to the Earth's surface, that transmits information from which an observer's present location and/or time of observation can be determined. Two operational systems, each of which qualifies as an SATPS, are the Global Positioning System and the Global Orbiting Navigational System.
The Global Positioning System (GPS) is part of a satellite-based navigation system developed by the United States Defense Department under its NAVSTAR satellite program. A fully operational GPS includes up to 24 satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55.degree. relative to the equator and being separated from each other by multiples of 60.degree. longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Theoretically, three or more GPS satellites will be visible from most points on the Earth's surface, and visual access to two or more such satellites can be used to determine an observer's position anywhere on the Earth's surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. Internal clock correction is provided for each satellite clock.
An SATPS antenna receives SATPS signals from three or more (preferably four or more) SATPS satellites and passes these signals to an SATPS signal receiver/processor, which (1) identifies the SATPS satellite source for each SATPS signal, (2) determines the time at which each identified SATPS signal arrives at the antenna, and (3) determines the present location of the SATPS antenna from this information and from information on the ephemerides for each identified SATPS satellite. The SATPS signal antenna and signal receiver/processor are part of the user segment of a particular SATPS, the Global Positioning System, as discussed by Tom Logsdon in The NAVSTAR Global Positioning System, Van Nostrand Reinhold, 1992, pp. 33-90, incorporated by reference herein.
Each GPS satellite transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f1=1575.42 MHz and an L2 signal having a frequency F2=1227.6 MHz. These two frequencies are integral multiples of f1-1540 f0 and f2=1200 f0 of a base frequency f0=1.023 MHz. The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and the P-code. The L2 signal from each satellite is BPSK modulated by only the P-code. The nature of these PRN codes is described below.
One motivation for the use of two carrier signals L1 and L2 is to allow partial compensation for propagation delay of such a signal through the ionosphere, which delay varies approximately as the inverse square of signal frequency f (delay f.sup.-2). This phenomenon is discussed by MacDoran in U.S. Pat. No. 4,463,357, which discussion is incorporated by reference herein. When transit time delay through the ionosphere is determined, a phase delay associated with a given carrier signal can be determined.
Use of the PRN codes allows use of the plurality of GPS satellite signals for determining an observer's position and providing navigation information. A signal transmitted by a particular GPS signal is selected by generating and matching, or correlating, the PRN code for that particular satellite. All PRN are known and are generated or stored in GPS satellite signal receivers carried by ground observers. A first PRN code for each GPS satellite, sometimes referred to as precision code or P-code, is a relatively long, fine-grained code having an associated clock or chip rate of 10 f0=10.23 MHz. A second PRN code for each GPS satellite, sometimes referred to as a clear/acquisition code or C/A-code, is intended to facilitate rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser-grained code having a clock or chip rate of f0=1.023 MHz. The C/A code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code for a given GPS satellite has the length of precisely one week (7.000 days) before this code portion repeats. Accepted methods for generating C/A-code and P-code are set forth in the document GPS Interface Control Document ICD-GPS-200, which is provided from Rockwell International Corporation, Satellite Systems Division, Revision A, 26 Sep. 1984, which is incorporated by reference herein.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite and an almanac for all GPS satellites, with parameters providing corrections for Ionospheric signal propagation delays suitable for single frequency receivers and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 BAUD. A useful discussion of the GPS and techniques for obtaining position information from the satellite signals is found in Tom Logsdon, The NAVSTAR Global Positioning System, Van Nostrand Reinhold, New York, 1992.
A second configuration for global positioning is the Global Orbiting Navigation Satellite System (GLONASS), Placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS also uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8.degree. relative to the equator, and the three planes are separated from each other by multiples of 120.degree. longitude. The GLONASS circular orbits have smaller radii, about 25,510 kilometers, and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS Satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. The GLONASS system uses two carrier signals L1 and L2 with frequencies of f1=1.602+9k/16) GHz and f2=1.246+7k/16)GHz, where k(=0, 1, 2, . . . , 23) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1) and 1,240-1.260 GHz (L2). The L1 code is modulated by a C/A-code (chip rate=0.511 MHz) and by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each satellite. The methods for receiving and analyzing the GLONASS signals are similar to the methods used for the GPS signals.
Reference to Satellite Positioning System or SATPS herein refers to a Global Positioning System, to a Global Orbiting Navigation System, and to any other compatible satellite-based system that provides information by which and observer's position and time can be determined, all of which meets the requirements of the present invention.
A satellite Positioning System (SATPS), such as the Global Positioning System (GPS) or the Global Orbiting Navigation System (GLONASS), uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. A single passive receiver of such signals is capable of determining receiver absolute position in an Earth-centered, Earth-fixed coordinate reference system utilized by SATPS.
U.S. Pat. No. 5,101,416, titled Multi-Channel Digital Receiver For Global Positioning System by Fenton et al. and assigned to NovAtel Communications Ltd. discloses a receiver for pseudo random noise (PRN) encoded signals consisting of a sampling circuit and multiple channel circuits. Each channel circuit includes a carrier and code synchronizing circuit and two digital correlators with dynamically adjustable code delay spacing. The two correlators compare the digital samples with a locally generated PRN code to produce early, late, and/or punctual correlation signals which are used to adjust the local PRN code.
U.S. Pat. No. 4,550,414, titled Spread Spectrum Adaptive Code Tracker by Guinon et al, and assigned to Draper Laboratory discloses a pseudo-noise code-tracking spread spectrum receiver able to quickly acquire and track incoming signals. The receiver includes an adaptive weighting system for the outputs of parallel-fed correlator channels in which the weighting system automatically detects the degree of correlation in each channel, sets the detector characteristics to increase the weight for a correlator channel exhibiting a high degree of correlation, and decreases to zero the weights on all others.
A "channel" for a GPS receiver is defined as a circuit which is capable of taking the digital output samples of the sampling analog-to-digital converter (ADC) and processing the signal transmitted from a satellite which is presently in view. The signal which is digitally sampled includes an in-phase (I) signal and a quadrature (Q) signal. The channel process the samples I and Q signals transmitted by a particular satellite. Each channel uses a carrier/code synchronizing circuit which tracks the frequency and phase of a PRN encoded carrier signal by using a expected Doppler offset unique to a desired satellite. A channel also correlates a locally generated Pseudo-Random code reference signal with a Doppler-rotated replica of the carrier. Two correlators are connected with a delay-clock arrangement to keep the locally generated PRN code precisely aligned with the code modulated onto the received satellite signal. The resulting decoded data includes the ephemeris of the satellite, time of day, status information, and the locally generated PRN code phase and carrier phase measurements.
A typical multi-channel module for a GPS receiver typically includes six channel circuits. Each of the channel circuits includes a carrier and code synchronizing circuit as well as multiple digital correlators. The correlators compare the digital samples with a locally generated PRN code to produce early, late, and/or punctual correlation signals which are used to adjust a local PRN code.
A GPS receiver typically uses 4 or more satellite signals for navigation. For acquisition, a GPS receiver may use one or more satellite signals during acquisition or re acquisition. This means that when fewer satellites are being used, there are usable channels that are idle, unused channels. Consequently, the need has arisen for more efficient use of these unused channels in a GPS system.